1. Field of the Invention
The present invention relates to a laminate circuit board comprising a plurality of laminated wiring layers and, more particularly, to a laminate circuit board which permits constants of various circuit elements contained therein to be set by selecting various combinations of connections between the wiring layers, and a laminate circuit board which permits distributed constants of a high-frequency wiring layer sandwiched between two shielding wiring layers to be set as desired.
2. Description of the Related Art
Conventionally, a laminate circuit board is formed by laminating a plurality of wiring layers formed with respective wiring patterns thereon, one upon another, and forming vias between the wiring layers when connections are required therebetween.
Such a laminate circuit board has been utilized not only as a mere laminate of substrates on which are arranged ordinary circuit elements and wirings connecting them, but also as one which forms a capacitor element, a resistance element, an inductance element, or the like, depending on the state of laminated layers.
FIG. 21 (A) and FIG. 21 (B) show the construction of a conventional laminate circuit board used as a capacitor element. FIG. 21 (A) is an exploded perspective view of the conventional laminate circuit board, while FIG. 21 (B) is a view showing same in its finished state. This capacitor element is a laminate capacitor 384 in which wiring layers 381 and 382 opposite in polarity are laminated in a plurality of pairs, one upon another. Bodies 381a and 382a of each pair of the wiring layers 381 and 382 are formed of dielectric material, and have surfaces thereof printed with electrode foils 381b and 382b, respectively.
The electrode foils 381b and 382b are printed such that they extend up to edges 381c and 382c which correspond to one ends of each of the bodies 381a and 382a of the wiring layers located on respective sides opposite each other. The wiring layers 381 and 382 are laminated in a required number of pairs, and then subjected to press and firing. On the laminate capacitor 384 formed by a press are mounted electrode terminals 384a and 384b on respective electrode take-out sides as shown in FIG. 21 (B), and the resulting laminate capacitor 384 is mounted on an electronic circuit as the capacitor element. The electrode terminals 384a and 384b are connected to the edges 381c and 382c, respectively.
In the laminate capacitor 384 constructed as above, the electrostatic capacity between wiring layers depends on the dielectric constant of dielectric material forming the bodies 381a and 382a and thickness thereof, and the areas of the electrode foils 381b and 382b. Further, it is possible to realize various values of a total electrostatic capacity of the laminate capacitor 384 by varying the number of laminated layers.
However, such a laminate capacitor 384 is required to take into consideration the number of wiring layers laminated, outer dimensions of same, etc. to meet a required value of electrostatic capacity, which results in a low manufacturing efficiency and poor manageability. Further, as the number of wiring layers laminated and the outer dimensions of same are changed, it is required to supply electrode terminals 384a and 384b having sizes corresponding thereto, this also degrades the manufacturing efficiency, etc.
Further, such a laminate circuit board constructed as a resistance element or as an inductance element other than one as the capacitor element described above also suffers from similar problems.
Further, when inductance element is formed by the laminate circuit board, constants of the inductance element are restricted by the material forming the bodies of wiring layers, which makes it impossible to obtain an inductance element having desired constants (especially, a constant of dielectric loss). If an LC filter, for example, is constructed by the use of such an inductance element, there arises a problem that a desired Q (quality factor) cannot be obtained.
On the other hand, a high-frequency circuit board using a strip line can be formed with patterns (of transmission lines) by IC technology and the like. Such a high-frequency circuit board is widely applied to portable telephones and the like because it is excellent in mass productivity, reproducibility, and economy, and small in size and weight.
The high-frequency circuit board of this kind is required to inhibit electric wave from leaking therefrom and prevent extraneous interferences from occurring thereto, as well as to facilitate adjustment of distributed constants thereof.
FIG. 22 shows a high-frequency circuit board of an open type using a typical micro strip line. This high-frequency circuit board comprises a dielectric substrate 401 formed of alumina ceramic, quartz, sapphire or the like, on one side of which is formed a strip line (conductor pattern) 402, and on the other side of which is formed a ground pattern (grounding conductor) 403.
Since the high-frequency circuit board is of an open type having the strip line 402 formed on the surface of the substrate, it is convenient for adjusting circuit characteristics, such as impedance, or for effecting maintenance. On the other hand, however, there arises problems of leakage of electric waves, extraneous interference, etc.
To solve these problems, a high-frequency circuit board (triplate strip line) of a laminate type has recently come into use, in which a central conductor (strip line) is shielded by grounding conductors arranged above and below the central conductor.
The high-frequency circuit board of this type is, as shown in FIG. 23, comprised of a strip line 404 as the central conductor, ground patterns 406a and 406b arranged on opposite sides thereof with dielectric layer (substrate) 405 interposed therebetween, and a surface wiring pattern 407a and a surface ground pattern 407b arranged on one surface thereof with the dielectric layer 405 provided thereunder.
The strip line 404 is electrically connected to the surface wring pattern 407a by way of a via 408a, while the ground patterns 406a and 406b are electrically connected to the surface ground pattern 407b by way of a via 408b.
The ground patterns 406a and 406b provided in pair are formed of plain flat foils, and the strip line 404 is shielded by the ground patterns 406a and 406b. Therefore, compared with the high-frequency circuit board of an open type described above, this high-frequency circuit board of a shielded type is markedly lower in leakage of electric wave and in extraneous interference.
However, in the conventional high-frequency circuit board of the shielded laminate type, a strip line and ground patterns are arranged in inner layers of the laminate circuit board, which brings about problems of difficulty in adjustment of circuit characteristics, such as impedance and matching of frequencies.